The present invention relates to a micromachine implemented by MEMS or the like, and a manufacturing method therefor.
One of techniques of implementing a micromachine is MEMS (MicroElectro Mechanical System) (reference 1: Japanese Patent Laid-Open No. 2001-198897, reference 2: Japanese Patent Laid-Open No. 2002-189178, and reference 3: “MEMS: Micro Technology, Mega Impact” Circuit & Device, pp. 14–25 (2001)). MEMS elements realized by MEMS are a switching element which is electrically turned on/off, and an optical switching element which enables/disables an optical signal. In a switching element, a small actuator is formed from silicon or a metal. The actuator is driven by an electrostatic force generated by an electrode which is arranged to face the actuator.
For example, there is a MEMS element shown in FIG. 5A. The MEMS element comprises a silicon actuator 804 which is supported by a support member 803 on a silicon substrate 801 having an insulating film 802. In this element, the actuator 804 is driven by an electrostatic force generated by an electrode 805 arranged on the substrate 801 below the distal end of the actuator 804.
In an optical switching element as one of MEMS elements, a mirror formed from a silicon substrate or the like is driven by an electrostatic force generated by an electrode arranged below the mirror. The optical switching element is constituted as shown in, e.g., FIG. 6. The optical switching element comprises a conductive support member 920 on an interlayer dielectric film 905 formed on a semiconductor substrate 901. The optical switching element also comprises a mirror substrate 930 which is supported by the support member 920 and has an opening region. A mirror 931 is pivotally arranged in the opening region of the mirror substrate 930. A control electrode 940 for pivoting the mirror 931 is arranged on the interlayer dielectric film 905 below the mirror 931. The control electrode 940 and support member 920 are connected to a wiring layer 904 arranged below the interlayer dielectric film 905.
For example, in the MEMS element shown in FIGS. 5A and 5B, it is not easy to attract the actuator 804 to the electrode 805 by an electrostatic force generated by the electrode 805 and keep the actuator 804 still at an arbitrary distance from the underlying electrode 805. This is because the balance between an attractive force of attracting the actuator 804 to the electrode 805 and an elastic force of returning the actuator 804 to an original position is unstable and easily lost. If the balance is lost and, e.g., the attractive force becomes stronger, the distal end of the actuator 804 comes into contact with the surface of the electrode 805.
If the actuator 804 is made of a conductive material and electrically connected to the electrode 805 upon contact, they react with each other and are jointed at a contact portion 806, as shown in FIG. 5B. The actuator 804 may not return to an original position by repulsion of its elastic force. This phenomenon is called sticking or fixation, and poses a problem in driving the actuator of a micromachine. Contact between the actuator and the electrode upon application of a high voltage is identical to so-called spot welding. The cause of this phenomenon is therefore supposed to be a kind of resistance welding.
The optical switching element shown in FIG. 6 also suffers this phenomenon. In this optical switching element, it is not easy to attract one end of the mirror 931 to the control electrode 940 by an electrostatic force generated by the control electrode 940 and keep the mirror 931 still at an arbitrary distance from the underlying control electrode 940. This is because the balance between an attractive force of attracting the mirror 931 to the control electrode 940 and an elastic force of returning the mirror 931 to an original position is unstable and easily lost. If the balance is lost and, e.g., the attractive force becomes stronger, the lower surface of the mirror 931 comes into contact with the end of the control electrode 940.
If the mirror 931 is made of a conductive material and electrically connected to the control electrode 940 upon contact, they react with each other and are jointed at a contact portion 950, as described above. The mirror 931 may not return to an original position by repulsion of its elastic force.
To avoid the sticking phenomenon, at least one contact surface is rendered nonconductive. For this purpose, an organic thin film or the like is formed on an electrode.
For example, before the mirror substrate 930 having the mirror 931 is arranged on the support member 920, an organic material is applied to the interlayer dielectric film 905 having the control electrode 940 and support member 920, thus forming an organic film which covers the control electrode 940. The organic film is also formed on the support member 920 upon coating, and an unnecessary portion must be removed by forming a photosensitive organic film and patterning it by known photolithography.
A complicated three-dimensional structure as shown in FIG. 6 is patterned by photolithography using ultra-deep exposure. Formation of an organic film which covers the control electrode 940 requires many photomasks. A micromachine is greatly corrugated, and the step coverage of a coating film becomes poor in applying an organic material and forming a film. Such poor step coverage may inhibit formation of an organic film in a region where an organic film should be formed, such as a region above the control electrode.